Is the levelling datum for a continental levelling network so stable that it would permit the determination of secular movements as accurate as modern precise levellings may be observed?

Abstract

In general the observations within a continental levelling network have been made during day time when the sun is above the horizon. In Northern countries levelling observations have often been made during the summer months and in the morning and the evening, when the sun may be to the North of the prime vertical. This entails that special mean tidal perturbations by the sun on a levelling network may deviate with not quite negligible quantities from general mean tidal influence by the sun on the sea. For the moon the corresponding deviation will be nearly zero. The variance of the levelling (REUN 1960) between the tidal stations M-28 Fredericia and M-48 Genova is (±33 mm x kiloGal)². The author (1965) has found for this line (hypothetical levelling 19/5–2/6 1950 on the Yielding Earth)

possiblespecial mean tidal correction by the moon +27.5 mm

possiblespecial mean tidal correction by the sun −4.2 mm

―

in total +23.3 mm

deviating 22.5 mm

fromgeneral mean tidal correction of the seafor both moon and sun} +45.8 mm

The deviation 22.5 mm between tidal corrections for Mean Sea Level, MSL, and for levelling line is not quite negligible.